The present invention relates to a double-walled quartz-glass tube for semiconductor-technology processes and comprises an inner tube surrounded by a coaxially positioned outer tube with an annular space left between them and with the ends of the tubes that face in the same direction each having a closure, whereby a processing-gas supply connection is mounted on the inner-tube closure and extends tight through the outer-tube closure and whereby the outer tube is sealed tight at the other end against the outside of the inner tube and has connections for supplying and removing a scrubbing gas.
Double-walled quartz-glass tubes of this type are discussed by Paul F. Schmidt in "Furnace Contamination and its Remedies," Solid State Technology, June 1983, pp. 147 ff. and by R. Krishnamurthy, B. E. Ramachandran, and K. Kaliyamurthy in "Contamination Control Using HCl Gas; Effects on Silica Glass and Minority Carrier Lifetime," Solid State Technology, August 1985, pp. 213 ff.
An extremely pure atmosphere is necessary for precipitating silicon and for melting and doping semiconductor pieces in a processing tube. Contamination, especially with certain metals but also with phosphorus, has to be prevented to the greatest extent possible. Especially critical contaminants are alkalis, the elements of the IIIrd and IVth group of the periodic system in general, copper, and heavy metals. The processing tubes are made out of quartz glass, which is especially satisfactory because of its high purity and corrosion resistance. Of the alkali metals, especially undesirable during the high-temperature processes involved in semiconductor technology is sodium, which is always present to a certain extent. Aluminum ions have a powerful getter effect on metal ions, which accordingly lose their inherent motility and move only at a very low diffusion rate. Thus, building aluminum ions into the outer surface of the quartz-glass processing tubes to obtain MOS's (metal-oxide semiconductors) with good electric properties is known. These layers also block the diffusion of sodium. Increasing the cleanliness of the sources of heat, which are another source of contamination, has also been attempted.
Since constantly increasing demands are being made on the quality of the semiconductor chips produced in these quartz-glass processing tubes, attempts have been made to employ double-walled quartz-glass diffusion tubes, flushing the space between the two tube components with a scrubbing gas during the processing in order to bond and remove the ions that diffuse through the wall of the outer tube.
A considerable rejection rate is typical, however, of the aforesaid known double-walled diffusion tubes. This rejection is due to fractures and cracks and to the breaking off of the supply and removal connections, especially the connections that supply processing gas to the inner tube. These defects derive from differences in the expansion of the inner and outer tubes.